When we use the SDMA in the UART driver(such as imx6q), we will
meet one situation:
  Assume we set 64 bytes for the RX DMA buffer.
  The RX DMA buffer has received some data, but not full.
  An Aging DMA request will be received by the SDMA controller if we enable the
  IDDMAEN(UCR4[6]) in this case.

So the UART driver needs to know the count of the real received bytes,
and push them to upper layer.

Add two new fields to sdmac, and update the `residue` in sdma_tx_status().
Signed-off-by: NHuang Shijie <b32955@freescale.com>
Signed-off-by: NVinod Koul <vinod.koul@linux.intel.com>